This invention relates generally to a method and apparatus for hemming a peripheral flange of an outer steel panel over a peripheral edge of an inner steel panel then laser welding the resulting hemmed joint.
Hemmed joints are well known in the automobile industry and, among other things, are used to join together inner and outer metal panels of doors and other closure members of automotive vehicles. Resulting hem joints each generally include a flange of metal of one panel being folded over onto an edge of another panel. Typically, in automotive vehicle construction, a peripheral flange or outer edge region of an outer panel is folded over and onto an outer edge region of an inner panel. The resulting hemmed joint provides a finished edge and a mechanical connection between the two panels that adds strength and rigidity to the member.
Tighter tolerance requirements in the automobile industry have made the fit and finish of door and other closure members on an automobile body increasingly important. For example, hinges that attach a door to an automobile body are physically attached to an inner door panel of the door. At the same time, the fit of the door in the automobile body is judged by the alignment of the outer panel relative to a door opening that the door must fit in when closed. Unfortunately, any misalignment between the outer edge of the door and the inner edge of the door opening is readily apparent. To achieve the desired alignment it""s not enough that the inner and outer panels be dimensioned to fit exactly as intended in the door opening. The inner and outer panels must also be accurately aligned relative to each other before they""re hemmed together and must maintain that alignment during and after hemming. After hemming, the hemmed joint around the periphery of the door will not allow the panels to completely separate from one another, but it""s still possible for the inner panel to shift relative to the outer panel. This is because the hemmed joint between the two panels is, in effect, only a slip joint.
To prevent slippage of the inner and outer panels relative to one another, the industry has focused on achieving what""s commonly referred to as xe2x80x9ctotal panel lockupxe2x80x9d. Total panel lockup is a condition achieved where, by hemming two panels together, the panels are effectively prevented from moving relative to one another. One known method for achieving this is to provide induction-cured adhesive in the hem joint. The adhesive is then cured in an oven or some other form of treatment chamber immediately after the hem is formed. However, physically moving an assembled door to an oven or other treatment chamber for adhesive curing can cause the panels to slip relative to one another. Using an instant-setting adhesive can solve this problem. An instant-setting adhesive will lock the panels together as soon as they contact one another. However, the use of such an adhesive would make it impossible, as is often necessary, to shift the two panels into their desired relative position after they""ve come into contact with one another.
It would, therefore, be desirable to achieve total panel lockup as quickly as possible after, but not before, a hemmed joint has been made. An example of a system that achieves total panel lockup after hemming is disclosed in U.S. Pat. No. 5,897,796 issued Apr. 27, 1999 to Forrest which disclosure is incorporated herein by reference. Forrest discloses a hemming and in-situ laser welding apparatus that includes a lower die shaped to support an outer metal panel in a position to be hemmed over outer edges of an inner metal panel. An upper die is mounted on a movable support for reciprocal motion between a raised position remote from the lower die and a down position adjacent the lower die. The upper die hems the outer panel by folding a peripheral flange of the outer panel over onto a peripheral edge region of the inner panel when the upper die moves into the down position. An inner panel hold-down and locating mechanism positions and holds together the two panels and establishes a stable condition for hemming. After the upper die has moved upward out of its down position, a plurality of laser units move in to weld the peripheral flange of the outer panel to the peripheral edge region of the inner panel. The Forrest in-situ welding system must pause, therefore, for the time it takes the upper die to move a sufficient distance upward from its down position.
It""s also known in the art to use spot welding to secure hemmed joints. However, spot welds typically require that the Class A surface of the outer panel be finished following spot welding. Finishing is necessary to obliterate distortions that spot welding causes on the outer surfaces of welded panels where spot welding electrodes are positioned during spot welding. In addition, metal panels can lose their relative orientation in the process of being transported from a hemming machine to a spot welding machine.
It""s also known in the art to laser weld a hemmed joint after removing the hemmed joint from a hemming press. Advantages of laser welding over conventional resistance welding include lower distortion in the welded panels, smaller heat effective zones, improved mechanical and structural performance, faster processing speeds, and elimination of the need to gain access to the weld site on a workpiece from both sides of the workpiece. To prevent separation of the panels that would preclude an effective weld, laser welding systems are also known to include welding clamps that hold together panels to be welded. Robots are also known to be used in laser welding operations to position and aim a laser emitter and/or laser director mirrors.
Also known in the art is the formation of conduction welds by heating the surface of a workpiece with a laser beam until the surface melts. According to this process, after the surface melts, heat conduction into the material melts a zone beneath a projection pattern of the laser beam. This zone is roughly hemispherical because heat loss is greater at the edge of the laser projection pattern than at its center. Consequently, so long as lasing is not allowed to continue long enough to start vaporizing the surface metal, conduction welds produce weld nuggets that aren""t very deep. As shown in FIG. 1, a properly formed conduction weld 60 has a weld nugget 66 with a cross section that""s wider than it is deep. While it""s possible to lase the surface of an upper metal panel long enough to form a large melt pool in the panels to be welded, the practical limit of penetration depth into the panels is about 1 mm. YAG lasers are most commonly used to produce such conduction welds although CO2 lasers are also known to be used.
Also shown in FIG. 1 is what""s known in the art as a keyhole weld 68 which forms when lasing is continued long enough to cause surface metal vaporization to occur during the formation of a conduction weld. The vaporized metal generates a channel that penetrates more deeply into a workpiece, creating fusion zones and therefore a weld nugget 70 that""s much deeper than it is wide. Keyhole welds are therefore undesirably deeper, rougher and more variable than conduction welds. The vaporized metal resulting from keyhole welds also generates more contamination than is found in conduction welds. Keyhole welds are what typically result from the use of YAG lasers.
Another type of laser that can be used in welding is the diode laser. Diode lasers include nearly microscopic chips of Gallium Arsenide or other exotic semiconductors to generate coherent light.
It would be desirable to provide a hemming and in-situ laser welding method and apparatus that welds a flange of an outer panel to an edge region of an inner panel immediately after hemming the panels together, and while an upper die of the apparatus is still in its down position after hemming.
The invention is a hemming and in-situ laser welding apparatus for hemming peripheral flanges of an outer steel panel over peripheral edges of an inner steel panel and laser welding the resulting hemmed joints. The apparatus includes a lower die configured to support at least a portion of an outer metal panel in a position to be hemmed over an edge region of an inner panel. An upper die is mounted on a movable support for reciprocal motion between a raised position remote from the lower die and a down position adjacent the lower die. The upper die is configured and positioned to hem the outer panel by folding a flange of the outer panel over onto the edge region of the inner panel when the upper die moves into the down position. A laser unit is positionable to weld an outer edge of the flange of the outer panel to an upper surface of the inner panel while the upper die is still in its down position holding the two panels together.
The invention also includes a method for hemming peripheral flanges of an outer steel panel over peripheral edges of an inner steel panel and laser welding the resulting hemmed joints. According to this method, inner and outer panels are provided between upper and lower dies of a hemming apparatus. The apparatus then forms a hemmed joint by folding a flange of the outer panel over onto an edge region of the inner panel by moving the upper and lower dies together. An outer edge of the outer panel flange is then laser welded to an upper surface of the inner panel after moving the upper and lower dies together and before moving the upper and lower dies apart. The upper and lower dies are then moved apart to allow for removal of the joined panels.
Objects, features and advantages of this invention include providing a method and apparatus for total panel lockup of an assembly that comprises a hemmed joint, providing a method and apparatus that achieves total panel lockup in a desired aligned relationship or relative position and does so immediately after hemming and before the panels have had an opportunity to shift out of their aligned relationship, allowing for and providing laser welding immediately after the panels have been hemmed, preventing spring-back of the folded-over flange, i.e., preventing the folded-over flange from raising out of contact with the edge region of the inner panelxe2x80x94a condition that would otherwise prevent a satisfactory laser weld from forming, and to provide a method and apparatus for welding a hemmed joint of two aluminum panels in which the aluminum does not need to be cleaned before the welding operation is performed.
These and other objects of the invention will become apparent from the following detailed description in which the reference numerals used throughout the description correspond to numerals shown on the drawing figures.